A fourth explosion hit the edge of the old city, a Unesco World Heritage site that has been heavily damaged during more than two months of fierce fighting between rebels and government forces.

The government said the blasts were caused by opposition suicide bombers. The technique is common to al-Qaida-style jihadist groups, some of which are known to have entered Syria to fight against the regime of president Bashar al-Assad.

The Syrian government has always blamed the uprising on foreign terrorists, even though the revolt began as peaceful protests by ordinary citizens that turned violent after attacks by security forces.

The Syrian opposition denies any links to terrorists or any use of suicide attacks.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said at least 40 people were killed and around 90 wounded in the four blasts, most of them members of the regime forces. It said mortars also targeted the nearby political security department around the same time of the bombings.

The Syrian security official, however, said most of the casualties were civilians. Regime troops had killed two more would-be suicide bombers before they could detonate their explosives, he said on condition of anonymity in line with government regulations.

Syrian state TV showed the bodies of three men wearing army uniforms at the site of the explosions. One of them appeared to be wearing an explosive belt with a timer tied to his wrist.

A Sunni extremist group called Jabhat al-Nusra, or Victory Front, has claimed responsibility for previous bombings.

Rebels last week announced a push to capture Aleppo, where they have been fighting regime troops since July.

But the bloodshed also is increasingly spreading outside Syria's borders. At least three people, including a six-year-old boy, were killed in a house in neighbouring Turkey on Wednesday by a shell fired from inside Syria, according to Abdulhakim Ayhan, mayor of the Turkish town of Akcakale. Turkey's state-owned Anadolu Agency reported that angry townspeople marched to the mayor's office in protest at the deaths.

In Aleppo, footage broadcast on state-run Ikhbariya TV showed widespread damage around Saadallah al-Jabri Square, which also houses a famous hotel and a coffee shop that had been popular with regime forces. One building appeared to have been leveled to the ground. The facade of another was heavily damaged.

The station showed video of several bodies, including one being pulled from the rubble of a collapsed building. Rescue workers stood on piles of debris, frantically trying to pull out survivors.